Conventionally, the AC voltages are controlled by electromechanical breakers or electronic devices also called Solid State relays, hereinafter SSR. The electromechanical breakers do not offer a protection to the load and to the line and an information about the state of the AC line or the current flowing into the load; these performances are reached only if these devices are used jointly with other units based on thermal or magnetic principles, but always mechanical devices, or with normal fuses, anyway devices that in a plant must be added to the main breaker increasing dimensions, costs, and wiring complexity.
Then, all these electromechanical devices have anyway a short and not constant operating life caused by the mechanical construction of the opening/closing mechanism of the contacts, subjected to a derating performance, specially if installed in a plant with presence of noises or temperature and humidity stresses and vibrations. Furthermore, due to the increasing presence inside the electrical boxes of others electronic devices, it has become an increasingly common requirement to reduce the electromagnetic noise that appears during opening/closing of mechanical contacts.
Furthermore, the use of SSR switches, as far is concerning the electromechanical noise, is very critical because they have a high noise factor, and also the SSR do not warrant a better protection of the load against over currents because they operate slowly in case of short circuit failure or over currents detection. The SSR, again, can not be used in an electrical installation for monitoring the load efficiency or current flow if not properly connected to an external and added electronics.